Various mechanisms have been developed to reduce NOx emissions from lean-burning engines. One mechanism is a catalyst known as a NOx trap. The NOx trap is a catalytic device typically positioned downstream of the catalytic converter in an emissions system, and is configured to retain NOx when the engine is running a lean air/fuel mixture for eventual reduction when the engine runs a more rich air/fuel mixture. A typical NOx trap includes an alkali or alkaline metal, such as barium or calcium, to which NOx adsorbs when the engine is running a lean air/fuel mixture. The engine can then be configured to periodically run a richer air/fuel mixture to produce carbon monoxide, hydrogen gas and various hydrocarbons to reduce the NOx in the trap, thus decreasing NOx emissions and regenerating the trap.
The use of a NOx trap can substantially reduce NOx emissions from a lean-burning engine. However, NOx traps are also susceptible to poisoning from sulfur in fuels, which may adsorb to the NOx adsorption sites in the form of sulfate (SO42−) or other oxidized sulfur compounds. These materials may be generally referred to as “SOx”, and may prevent NOx from adsorbing to trap surfaces, thereby impeding proper trap performance.
Various methods of desulfating (“deSOx”) NOx traps may be used. In general, these methods involve heating the NOx trap to a temperature sufficient to allow the reduction of SOx, and then producing a rich exhaust to reduce the SOx. However, it may be difficult to determine when trap performance has degraded sufficiently due to sulfur poisoning to perform a deSOx process. Furthermore, as the trap is aged thermally and/or chemically, the interval at which deSOx processes are needed may change over time, thereby contributing to the difficulty in determining when to perform a deSOx process.
The inventors herein have realized that desulfation may be more efficiently performed by following a method of controlling the engine, wherein the method comprises initiating a desulfation process in the NOx trap; measuring a delay between initiating the desulfation process and detecting a temperature increase in the NOx trap; and adjusting an engine operating parameter based upon the delay between initiating the desulfation process and detecting a temperature increase in the NOx trap. The engine operating parameter may be related to the timing of performing a subsequent deSOx process, and/or may be related to an engine operating condition used during a deSOx process.